Potential Reintroduction of VectorBorne Diseases

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Potential Reintroduction of Vector-Borne Diseases

• Rebecca J. Eisen, Ph.D.
• Division of Vector-Borne Infectious Diseases
• Centers for Disease Control and Prevention
• Fort Collins, CO

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Risk Assessment for Potential Reintroductions

• Is the pathogen of interest likely to be
  reintroduced?
• What factors are related to the likelihood of the
  pathogen being re-established following a
  reintroduction?
• Specific examples
• Yellow Fever
• Dengue
• Malaria
• Plague

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What are likely routes of pathogen reintroduction?

• Introduction of infectious vectors
• Introduction of infectious humans
• Introduction of infectious zoonotic hosts

4
Re-introduction Transit between the
south-eastern U.S. and climatically similar
disease endemic regions

• Air travel
• Cargo ships
• Exotic pet trade
• Migratory birds
• Intentional release

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Reintroduction Vector-specific factors

• Preferred breeding habitat of the vector is
  present at the site of origin
• Host preference
• Human-specific for maintenance in humans
• Catholic feeder that frequently bites humans
  (bridging vector)
• Vector efficiency
• Extrinsic incubation period
• Likelihood of surviving to the second bloodmeal
• Efficiency of transovarial transmission
• Ability to remain infectious long-term (e.g.,
  introduction of insects on ships)

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Reintroduction Vertebrate host-specific factors

• Duration of infectivity
• Duration of incubation period
• Pathogen load at or above transmission threshold
• Degree of virulence

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Vectorial Capacity Models
How many potentially infective mosquito bites
will ultimately be delivered by all vectors
feeding on a single host in 1 day?
m vector density in relation to the host a
probability a vector feeds on a host in 1 day
host preference index x feeding frequency b
the proportion of vectors ingesting an infective
meal that successfully become invective p
probability the vector will survive 1 day n
duration of the extrinsic incubation period (in
days) 1/(-lnp) duration of the vectors life,
in days, after surviving the EIP
Sir Ronald Ross
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Establishment Vector-specific factors

• Number of vector species
• Density of vectors
• Host preferences
• Human-specific for anthroponotic cycles
• Catholic feeders for zoonotic pathogens
• Specific to non-human hosts to maintain zoonotic
  cycles
• Seasonal host switching
• Seasonal patterns of vector populations
• Vector efficiency

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Establishment Vector-specific factors

• Length of the extrinsic incubation period
• Duration of infectivity (e.g., overwintering)
• Transovarial transmission
• Length of gonotrophic cycle
• Likelihood of surviving to second bloodmeal
• Infection rate in vector population
• Co-infections in vectors (and effect of secondary
  infection on transmission)

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Establishment Host-specific factors
Susceptible
Exposed
Infectious
Recovered

• Density of susceptible hosts (human or zoonotic)
• Influenced by cross-immunity
• Duration of immunity can infection burn out in
  small host pool? Herd immunity?
• Reproductive rate of host (new susceptibles in
  population)
• Density of infectious hosts (incidence)
• Mortality/virulence
• Likelihood of contact between infectious and
  susceptible hosts/vectors
• For humans peridomestic vs. recreational risk?
  Time of year?
• For non-human hosts seasonality will play a
  large role

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Establishment Host-specific factors

• Duration of infectivity (overwintering),
  virulence
• Duration of incubation or latency period
• Pathogen replication rate and time to
  transmissible level
• Ability of host to support pathogen load at or
  above the transmission threshold long enough to
  transmit
• Mobility of host during infectious period
• Single or multiple host species

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Establishment Host-specific factors

• For zoonotic hosts, what taxa or guild?
• Avian
• frequently attracted to peridomestic environments
• increased chance of contact with humans
• Non-human primate
• Not likely to establish in the southeastern US
• Livestock
• Small mammals

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Establishment climate and landscape factors

• Is the southeastern U.S. climatically and
  ecologically similar to regions of endemicity?
• Type of breeding environment
• Vegetation type
• Growing degree days
• Mean, maximum, and minimum temperatures
• Is the southeastern landscape suitably connected
  to promote transmission?
• If livestock serve as amplifying hosts, are
  agricultural areas connected (via transmission
  pathways) to urban areas?
• Are zoonotic hosts and humans connected in the
  landscape?

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Malaria

• Malaria is a mosquito-borne protozoan infection.
  Plasomdium falciparum and P. vivax, the two most
  common species causing disease in humans, are
  maintained in anthroponotic cycles.
• Incidence of disease is highest in sub-saharan
  Africa
• Life cycle is strongly temperature dependent. To
  complete the life cycle in Anopheles mosquitoes,
  temperatures must be gt20C (often absent in
  highlands within endemic regions)
• In southeastern U.S. Anopheles quadrimaculatus is
  a competent vector
• Malaria was eliminated from the continental US by
  1950
• Population shift from rural to urban
• Improved water management, housing and access to
  medical services
• Improved vector control, case finding and
  treatment
• Malaria is the most commonly imported
  vector-borne disease with approximately 1,200
  reported annually
• Reports of limited local transmission
• New Jersey (1991), New York (1993 and 1999),
  Texas (1993), Michigan (1995), and Georgia (1999)

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Dengue

• Mosquito-borne (Aedes aegypti) flavivirus
  maintained in anthroponotic cycles
• Denguelike illness was first reported in North
  America during an outbreak in Philadephia in
  1780.
• Locally-acquired cases have been rare in the US
  in the past 50 years
• From 1977 to 1995, more than 2,706 suspect and
  584 confirmed dengue cases were reported from
  travelers to endemic areas
• Dengue is endemic in Mexico
• From 1980-1999 62,514 dengue cases were reported
  from Mexican states bordering texas, whereas only
  64 locally-acquired cases were reported in Texas
  during the same time period (Reiter et al. 2003)
• Incidence of disease affected by human behavior
• Vector abundance was higher in Texas
• Air conditioning, evaporative coolers, intact
  screens, greater distance between homes and fewer
  residents per household was protective

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Yellow Fever

• Mosquito-borne (Aedes aegypti) flavivirus
  maintained in sylvatic cycles by non-human
  primates in sub-Saharan Africa and tropical South
  America
• Occurs seasonally coinciding with vector
  abundance
• Incidence typically higher in Africa than South
  America likely because in Africa there are more
  species of competent vectors that frequently bite
  non-human primates and humans and there is a
  lower rate of immunity in the human population.
• Epidemics occur when YFV is introduced into
  human-mosquito cycles in high density settings
• Last epidemic in North America occurred in New
  Orleans, 1905
• The virus was eliminated from the US through
  quarantine and mosquito control.
• Only 3 imported cases of Yellow Fever have been
  reported since 1970 (IHR require proof of YF
  vaccination for travelers)
• Aedes aegypti still abundant in southeastern US,
  but being displaced by the tiger mosquito Aedes
  albopictus
• Despite reintroductions, probability of
  re-establishment is low because of the absence of
  zoonotic hosts and low contact rates between
  humans and infected vectors

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Plague

• Plague is a severe, primarily flea-borne
  bacterial zoonosis caused by Yersinia pestis
• Identified in New Orleans in 1914 (244 of 378,563
  small mammals tested positive), following
  introduction via rat-infested cargo ships
  (continued maritime re-introductions through
  1934)
• From 1914-1920, a total of 51 human cases were
  reported of which 18 were fatal 1 additional
  case (stowaway) was reported in October 1924
• Xenopsylla cheopis implicated as primary vector
  (flea index as high as 18 in warmest months and
  rate of infection in rats increased with the flea
  index)
• Elimination of plague in New Orleans was
  attributed primarily to intensive rat-trapping
  and secondarily to rat-proofing and removal or
  rat harborage
• In an attempt to control plague, several
  ordinances were instated, resulting in several
  permanent sanitation improvements for the city
• rat proofing, garbage storage and collection,
  animal-holding regulations, much improved system
  of wharves including inspection and quarentine

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Conclusions

• Although devastating at the time, previous
  epidemics of Yellow Fever, Dengue, Malaria, and
  plague led to improvements in sanitation, public
  health, vector control and quarantine
• Provided such infrastructure is maintained or
  improved these vector-borne diseases are unlikely
  to re-emerge in the southeastern US

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Acknowledgments

• Ron Rosenberg
• Brad Biggerstaff
• Ken Gage
• Ben Beard

• Lars Eisen
• Barry Miller
• Chet Moore
• Erin Staples
• Tom Burkot